The fireworks display scheduled in Lansdale the last Saturday evening of this month has been postponed.   The idea behind it was to celebrate the 138th anniversary of the founding of the borough as well as cap off the activities for the annual Lansdale Festival of the arts.   Borough officials hope to reschedule the pyrotechnics before the end of the year and say it will still be held in Memorial Park.   Possible dates include New Year's Eve, an upcoming First Friday and the evening following the Mardi Gras Parade in November.